Accelerator, a Level 5 esper, resides at the forefront of Academy City's scientific and technological advancements, despite being severely weakened by a brain injury. He is accompanied by Last Order, a young girl he has sworn to protect at all costs. A mysterious young woman approaches Accelerator with intentions that put Last Order and her sisters in danger, prompting the highest-ranked esper to confront a powerful organization embedded within Anti-Skill, Academy City's peacekeeping force. This sets the stage for Accelerator to revisit the dark paths he once walked.